1. Field of the Invention
The present invention relates to a phase-type diffractive optical element that achieves various optical functions utilizing light diffraction phenomenon, and it also relates to a method of producing the same, and an optical apparatus.
2. Description of the Related Art
A diffractive optical element is a known optical element that utilizes light diffraction phenomenon to realize various optical functions such as a lens function (converging and diverging), wave demultiplexing/multiplexing function, a light intensity distribution converting function, a wavelength filtering function, and a spectroscopic function. In particular, a diffractive optical element that has a spectroscopic function is referred to as grating while a diffractive optical element that has a lens function is referred to as zone plane.
The phase-type diffractive optical element is made by forming an uneven pattern of a diffraction grating on the surface of an optical material (for example, optical glass). The form of the uneven pattern determines an optical function thereof. For example, a grating such as a blaze grating (FIG. 9) has an uneven pattern of an equi-pitch linear form. A zone plate (FIG. 10) has an uneven pattern of a concentric-circular form.
To simultaneously achieve a plurality of optical functions, a plurality of different types of diffractive optical elements each having a single function are disposed in series generally. According to this manner, it is possible to realize a plurality of optical functions of the diffractive optical elements comparatively easily.
However, disposing the diffractive optical elements each having a single function in series is not an effective way of production because for the assembly of an optical system composed of the diffractive optical elements it is needed to accurately align the diffractive optical elements with each other. In addition, the number of required diffractive optical elements increases as the number of functions to be realized increases, which makes it more difficult to produce diffractive optical elements with high accuracy and reduce the size of an optical system.
Thus, there has been proposed another method of forming a combined pattern on a surface of an optical material by combining uneven patterns corresponding to a plurality of intended optical functions. This method aims to realize a small diffractive optical element having multiple functions, however, it is difficult to accurately machine the pattern on the surface because of the complex shape of the combined uneven pattern.
To realize both of the lens function and spectroscopic function simultaneously, for example, an uneven pattern needed to be formed on the surface will be a combination of an equi-pitch linear pattern shown in FIG. 9 and a concentric-circular pattern shown in FIG. 10. The combining is performed by summing up phase modulation of each of the patterns. As shown in FIG. 11, the combined uneven pattern is very complex and their symmetricalness is low.